From the viewpoint of design properties, metallic decorative members, in particular, decorative members having mirror-surface like metallic luster have been widely used in the casing or button of a mobile phone; the casing of a watch; and the front grille, bumper or the like of a vehicle.
As such decorative members, a decorative member that transmits radio waves (microwaves or the like) without adversely affecting the radio waves has been required for the following reasons.
(i) An antenna for transmitting and receiving radio waves is disposed inside the casing of a mobile phone.
(ii) An antenna for receiving radio waves is disposed inside the casing of a radio controlled clock that is provided with a function to receive the standard wave and to automatically correct errors.
(iii) In a vehicle equipped with a radar device that detects the presence of obstacles or measures the distance between vehicles, an antenna in the radar device is disposed in the vicinity of a front grille or a bumper.
(iv) The frequency of radio waves used in the communication devices or the like has been shifting to a high frequency range of about 100 GHz, at which radio waves are readily affected by the decorative members, and thus these devices are prone to functional disorders.
The following materials have been proposed as the decorative members that allow the passing through of radio waves and are provided with metallic luster.
(1) A shaped product having a deposited film of indium, an indium alloy, tin, or a tin alloy, on a substrate (refer to Patent Document 1).
(2) A transfer material having a deposited film of an indium/indium oxide complex on a base material (refer to Patent Document 2).
(3) A decorative product having a coating film, in which fine pieces of luminescent materials are dispersed, on a base material (refer to Patent Document 3).
(4) A decorative product having a reflective film (metal) which is provided with an opening on a base material (refer to Patent Document 4).
With respect to the metal deposited film of indium, tin, lead, zinc, bismuth, antimony, or the like, it has been known that because the metal is present as a minute independent island, radio waves can pass through the gaps between the islands where no metal is present. For this reason, the decorative members described above in (1) and (2) have radio wave transmitting properties as well as metallic luster.
However, with respect to the decorative members of (1) and (2), if the thickness of the metal deposited film is increased in order to attain a satisfactory level of metallic luster, the islands become partially connected with each other and form a network that serves as a good conductor, and thus the reflection or absorption of radio waves occurs depending on the frequency thereof.
In addition, tin is prone to oxidation, chlorination, or the like, as a result of which the metallic luster thereof is lost over time. On the other hand, indium is highly expensive.
Because the above decorative member of (3) is a product in which luminescent materials are dispersed in a coating film, it is not provided with mirror-surface like metallic luster.
On the other hand, with respect to the above decorative member of (4), only radio waves having a specific frequency that conforms to the size of the opening of the light reflecting layer can pass therethrough.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2005-249773
[Patent Document 2] Japanese Patent No. 3414717
[Patent Document 3] Japanese Laid-Open Patent Application No. 2006-282886
[Patent Document 4] Japanese Laid-Open Patent Application No. 2006-276008